1. Field of the Invention
The present invention relates to an electric power steering device for employing an electric motor to assisting the steering force applied to road wheels. It further relates a method and apparatus for manufacturing the electric power steering device.
2. Discussion of the Related Art
Heretofore, there have been known electric power steering devices of the type that a steering mechanism connected to a steering shaft is given an assist force generated by an electric motor thereby to reduce the steering force to be applied to a steering wheel. In an electric power steering device of this kind, the steering wheel is rotated more than one full turn to the left and more than one full turn to the right within a limited rotational range. Thus, a neutral position of the steering wheel is assumed to be “the position that the steering wheel takes while the vehicle runs straight ahead”, and a steering angle is grasped by detecting as an absolute position the position to which the steering wheel is turned from the neutral position to the left or right.
And, the steering angle of the steering wheel is usually detected by the use of a rotary angle sensor which is composed of, e.g., a slit disc rotating with the steering shaft and a photo interrupter. For example, taking the configuration like “the steering sensor and the power steering device” disclosed on pages 2 to 8 and FIGS. 1 to 4 in Japanese unexamined, published patent application No. 2002-145095, it can be realized to precisely detect the neutral position of the steering wheel with a single rotary angle sensor.
By the way, steering wheels of vehicles are generally of the construction that the angle of steered wheels (usu., front wheels) over the whole range cannot be controlled through the steering within one rotation (360 degrees). For example, as the steering wheel is turned from the center or the neutral position two rotations (720 degrees) in the left direction and two rotations (720 degrees) in the right direction, the steering wheel is capable of varying its steering angle as desired within a predetermined angular range by rotating the steering wheel through 720 degrees toward the plus side or the minus side. For this reason, even where the same construction is taken as “the steering sensor and the power steering device” disclosed in the aforementioned Japanese application, it is impossible to detect the absolute rotational position of the steering wheel in addition to the rotational angle thereof by the use of one rotary angle sensor. Therefore, plural rotary angle sensors have to be combined in detecting the absolute rotational position. However, since in another aspect, the construction using the plural rotary angle sensors directly results in increasing the product cost as well as in heightening the frequency of trouble occurrence, such construction as to increase the number of components is difficult to adopt as a matter of the fact.
Another electric power steering device has also been known, wherein there are employed a resolver as means for detecting the rotational position of an electric motor which generates the assisting force and another resolver as means for detecting the steering torque generated by a steering wheel. However, it was found through researches of the present inventor that it may occur in depend no on some angular positions of the steering wheel that the latter known electric power steering device cannot precisely distinguish some absolute rotational positions from others precisely or definitely.